(1) Field of the Invention
The present invention relates to spoked vehicle wheels and bicycle wheels in particular. More specifically, this invention relates to the spoke bed geometry of the rims of such wheels.
(2) Description of the Related Art
In the development of a tension-spoked wheel, the geometry of interaction between the spoke and the rim is of particular importance as it relates to the strength, stiffness, and longevity of the completed wheel structure. The overlie engagement between the under-head surface of the nipple and the spoke bed of the rim served to provide the requisite bracing to resist the spoke tension forces of the wheel. Heretofore, the spoke bed of the bicycle rim is commonly designed to have either a flat or concave surface in the region where it contacts the under-head portion of the spoke nipple. The under-head geometry or transition portion of the spoke nipple is commonly a tapered conical surface.
It is noted that the spoke does not exit the rim in a perfectly radial direction. Instead, the spoke is directed with an angle of inclination toward one of two (or more) axially spaced hub flanges. Given the geometry just described, close inspection of the interface between the spoke bed and the rim reveals some significant shortcomings that can compromise the strength and stiffness of the wheel.
Firstly, it is shown that, due to this angle of inclination of the spoke, the under-head surface of the spoke nipple contacts the spoke bed at only a single contact point. Such a small contact area, combined with the high spoke tension of modern wheels, creates very high contact stress at this singular contact point. The result is excessive galling between the spoke nipple and the rim as the nipple is rotatably adjusted to bring the spoke up to the desired tension. This provides resistance to rotation of the nipple and thereby makes the nipple more difficult to adjust. In addition, this also causes the nipple and rim to abrade against each other, removing nipple and/or rim material and potentially weakening the structural integrity of one or both of these components.
Secondly, this contact point is offset from the centerline of the spoke. Since the spoke tension acts along the spoke centerline, and the resisting force acts at this contact point, this offset results in a bending moment at the spoke nipple. Since the spoke tension increases and decreases cyclically as the wheel is rotated, this bending moment introduces a bending stress to the spoke, which reduces the fatigue life of the spoke and/or nipple and/or rim. In fact, it is not at all uncommon for a spoke to fail due to fatigue under normal use.
Further, this bending moment tends to deflect the spoke and add a bent region in the spoke adjacent the nipple. The bent region will tend to flex somewhat due to the variations in spoke tension experienced during normal use of the wheel. This flex has the effect of reducing the effective tensile stiffness of the spoke and thus tends to reduce the lateral stiffness of the wheel. The result is a more “wobbly” and less responsive feel on the part of the rider. This bending also serves to increase fatigue stresses and exacerbate spoke failure due to fatigue.
There may be some cases where the spoke nipple may be constricted at its connection point at the rim such that the spoke nipple is held in perfect radial alignment, rather than self-aligning along the angle of inclination of the spoke. While this approach may reduce or eliminate the aforementioned offset, the result is that the spoke will instead be bent in a new region that is usually inboard of its connection at the rim. Again, the result is undesirable flex and fatigue of the spoke in this bent region.
In a rim with a concave outboard surface, the center of the outboard surface is radially inboard from axially spaced flanking outboard surfaces. With a radial spoke hole piercing a concave spoke bed the radial depth of the hole is deeper adjacent these outboard surfaces. This requires a larger spoke hole in order to accommodate the off-axis alignment of the nipple due the angle of inclination of the spoke. By definition, the larger spoke hole removes more material from the rim and therefore serves to weaken the rim.
Further objects and advantages of the present invention will appear hereinbelow.